Field of the Invention
The present invention relates to a wet friction disk. The present invention more particularly relates to a wet friction disk that is used in the presence of oil and that can be incorporated in a wet clutch, a wet brake, and the like.
Related Art
In the related art, wet clutches and wet brakes using a wet friction disk are used for torque transmission and braking. Specifically, wet clutches are used in automatic transmissions of vehicles and the like. Such a wet clutch has a function of transmitting driving power generated by a driving source such as an engine to a driven body such as an axle, and of shutting off the driving power. For example, in a case of using a wet friction disk shaped into a plate and associated with the driving source, and a separator plate associated with the driven body, the wet friction disk and the separator plate are arranged facing each other with a clearance interposed therebetween. Further, a lubricating fluid is supplied around the wet clutch and has a function of absorbing frictional heat caused between the plates and preventing wear in the plates. Then, in a state where a relative rotation of the two plates is performed while the two plates are spaced apart (idle running state), the driving power is not transmitted to the driven body. In a state where the two plates are pressed against each other, the driving power is transmitted from the driving source to the driven body.
In such a wet clutch, the clearance between the two plates is set to be small, for example, for purpose of improvement in responsiveness. However, since, in a state where the two plates are disengaged, the relative rotation of the two plates is performed while the two plates are spaced apart, a “drag torque” is caused depending on a amount of the lubricating fluid interposed between the plates and a lubricating state. In other words, there is a problem of unnecessarily consuming energy during the idle running because of generation of the drag torque. Thus, a reduction in the drag torque has been a challenge in recent years as part of rapid advancement of measures for the improvement of fuel efficiency.
A known technique to reduce the drag torque is described in JP-A 2009-052601.
A wet friction disk using a ring-shaped core plate having a wave shape in a circumferential direction thereof is disclosed in JP-A 2009-052601. This technique is excellent in that the use of a core plate having the wave shape in the circumferential direction enables a reduction in the drag torque in a wide range of rotation speed ranges in comparison with a core plate that does not have such a wave shape.
However, while a further reduction in the drag torque is demanded, especially a further reduction in the drag torque in a high rotation speed range is demanded in a case of using a core plate having a wave shape in the circumferential direction.